Hair growth begins with the production of keratinocytes by the basal cells of the hair bulb. As new cells are deposited at the hair bulb, the hair shaft is pushed through the follicle toward the surface. Keratinization is completed as the cells are pushed to the skin surface to form the shaft of hair that is externally visible. The external hair is completely dead and composed entirely of keratin. Hair can be cut or shaven without damaging the hair structure because the cut is superficial. Most chemical hair removers also act superficially; however, treatments like electrolysis and laser hair removal attempt to destroy the hair bulb so hair cannot grow.
Hair grows and is eventually shed and replaced by new hair. This occurs in three phases. The first is the anagen phase, during which cells divide rapidly at the root of the hair, pushing the hair shaft up and out. The length of this phase is measured in years, typically from 2 to 7 years. The catagen phase lasts only 2 to 3 weeks and marks a transition from the hair follicle’s active growth. Finally, during the telogen phase, the hair follicle is at rest, and no new growth occurs. At the end of this phase, which lasts about 2 to 4 months, another anagen phase begins. The basal cells in the hair matrix produce a new hair follicle, pushing the old hair out as the growth cycle repeats itself. Hair typically grows at the rate of 0.3 mm per day during the anagen phase. On average, 50 hairs are lost and replaced per day. Hair loss occurs if more hair is shed than what is replaced and can happen due to hormonal or dietary changes. Hair loss can also result from radiation therapy, chemotherapy or the aging process.
Similar to the skin, hair gets its color from the pigment melanin, produced by melanocytes in the hair papilla. The wide range of hair color in humans results from differences in the type of melanin, which is genetically determined. As a person ages, melanin production decreases, and hair loses color and becomes gray and white.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 5.2: Accessary structures of skin.
